23 Sep 2020  |   04:51am IST

Wetlands of Goa: temporal evolution of khazans

Wetlands of Goa: temporal evolution of khazans

Antonio Mascarenhas

Of all the natural tropical ecosystems, the beach – dune complexes and the low-lying coastal wetlands – are the youngest geological units. In Goa, large stretches of open fertile plains, popularly known as ‘khazan lands’ which comprise paddy fields, are found often up to the base of the Western Ghats. Most of the historical documents describe these systems as areas reclaimed from the sea, a hypothesis that has prevailed since decades. However, from a scientific viewpoint, there is no evidence whatsoever that records the geological evolution of events that culminated in the formation of the khazans of Goa.

To test this ‘reclaimed land’ hypothesis and to study the geological history of the wetlands of Goa, we collected sediment samples at several locations from Carambolim in the north to beyond Cortalim in the south. The pits dug by the Konkan Railway served as ideal locations to collect sediment from the surface down to about 4 metres below the surface. All along a 15 km stretch from Carambolim to beyond Cortalim, peat occurs in the form of black clay with abundant vegetal debris. The most interesting find was a metre-long wood log buried in clay, recovered at Mandur. Peat occurs 1.5 to 2.5 m below surfaces which, at present, are almost at sea level. 

Sedimentological analyses reveal that organic peat samples are dominantly composed of 69 per cent clay fraction. Wood pieces and fragments were identified from all the sites. The reddish-brown log more than a metre long resembled an ancient tree stump, and was found to be full of burrows and encrusted with wood boring organisms. Peat is a sedimentary organic deposit derived almost exclusively from plant products. Humid climate, abundant precipitation, large source of plant debris, retardation of organic decomposition and low influx of inorganic sedimentary material are some of the prerequisites which promote vigorous plant growth, with consequent peat accumulation. In the tropics, mangroves are the most remarkable and the only intertidal trees which inhabit bays and lagoons influenced by sea water. These plant communities have been found to be the most prolific producers of peat. Autochthonous peats are markers of ancient forests, past sea levels and shorelines. 

Fine grained sediments of silty clay grade, with a dominant mineral smectite, can only be deposited in regions under calm conditions. Since smectite is a product of weathered volcanic rocks, found only along the coast of Maharashtra in the north, it is obvious that this mineral has been transported and introduced in the hinterland of Goa via the sea. This also means that this entire area was once directly connected to the sea. The only possible connection with the sea could be to the north but more certainly to the south where the present Cumbarjua canal opens in the river Zuari. Since the organic sediments are found at a depth of 1.5 to 2.5 m below ground level, it could be concluded that the study area once comprised a large intertidal zone, probably under a shallow water lagoon and a low energy marine environment. 

The organic peat with significant plant remains also indicate that the entire area became a site conducive for plant growth. Only mangroves could have colonized the area thus making it a luxuriant mangrove swamp. This is evidenced by a mangrove tree stump recovered from the peat unit. Moreover, the encrusted wood borers which belong to the category of “Martesia” only attack mangrove wood exclusively, and thus confirm the mangrove stump hypothesis. From the preceding discussion it can be postulated that the coastal plains of Goa were conducive for forest growth and were therefore the scene of widespread mangroves swamps during the recent geological past. 

Mangroves being intertidal are markers of past sea levels. Along the west coast of India, the sea level rose at a very high rate from 10,000 to 7,000 years BP, after which it almost stabilised. The horizons where peat was deposited are now found at 1.5 to 2.5 m below present sea level. Therefore, the paleo sea level was around 1 to 3 m below the present level, and the time when the sea level had reached this position during the geological past was between 7,000-6,000 years BP. Mangrove debris has also been identified below coastal plains near Karwar, Coondapur, Tellicherry and Changanacherry along the west coast; the age ranges from 7,230 to 6,000 years BP. Similarly, a plant rich horizon, 1.5 to 2.5 m below present sea level, was identified along the coast at Colva (Goa); fossilized wood and twigs within this black layer yielded an age of 6,430 years BP. Considering the proximity of this site, and since the organic debris identified in this study are also found at identical depths below present sea level, peat within wetlands of Goa may be assigned the same age, around 6,430 years BP.

In conclusion, (1) the geomorphic configuration of the coastal plains of Goa was altogether different during the geological past. (2) The sea level was 1 to 3 m lower and the climatic conditions were wet and humid. (3) Thick peat deposits prove that wetlands of Goa were once occupied by luxuriant and widespread mangrove swamps under shallow water, low energy, sheltered environments. (4) Subsequently lowlands were filled by lateritic detrital sediment when the sea level attained its present position. (5) The influence of man in reclaiming land cannot be ascertained with available scientific data, except the ‘bundhs’ of historical times.

Note: This article is based on a scientific paper entitled ‘A note on the ancient mangroves of Goa’ by Mascarenhas and Chauhan, published in the Indian Journal of Marine Sciences, in 1998. To our knowledge, further research that records the geo-history of the wetlands of Goa has never been attempted.  

(Dr Antonio Mascarenhas is a former Scientist, NIO, Goa)


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